Podophyllotoxin is a naturally occurring compound extracted from the mandrake plant. Recently two therapeutically useful semi-synthetic glycosides of podopyllotoxin, etoposide (also known as VP-16), shown below, and teniposide (also known as VM-26), have been developed. ##STR3## These compounds exhibit therapeutic activity in several human neoplasms, including small cell carcinomas of the lung, testicular tumors, Hodgkin's disease, Papillomavirus, and diffuse histiocytic lymphoma.
It is believed that these drugs block the catalystic activity of DNA topoisomerase II by stabilizing an enzyme-DNA complex in which the DNA is cleaved and covalently linked to the enzyme. See Chen, G. L., Yang, L., Rowe T. C., Halligan, B. D., Tewey, K., and Liu, L., J. Biol. Chem., 259, 13560 (1984); Ross, W., Rowe, T., Glisson, B., Yalowich, J., and Liu, L., Cancer Res., 44, 5857 (1984); Rowe, T., Kuppfer, G., and Ross, W., Biochem. Pharmacol., 34, 2483 (1985), which are all herein specifically incorporated by reference. By way of background, topoisomerases are enzymes which control the topological state of DNA. Type II topoisomerases catalyze DNA strand passage through transient double strand breaks in the DNA. The resulting change in the linking number of DNA allows these enzymes to mediate DNA interconversions, such as supercoiling and relaxation of supercoiling, catenation and decatenation, knotting, and unknotting. See Wang, J. C., Annu. Rev. Biochem., 54, 665 (1985) and Maxwell, A., and Gellert, M., Adv. Protein Chem., 38, 69 (1986), which are herein specifically incorporated by reference.
Type II DNA topoisomerase enzymes have been shown to be involved in a number of vital cellular processes, including DNA replication and transcription, and chromosomal segregation. These enzymes, therefore, are a critical target for the action of a wide variety of anticancer drugs, including etoposide and teniposide. The key step leading to cell death may be the capability of these drugs to block the catalytic activity of DNA topoisomerase II, as noted above.
Structure-activity studies have demonstrated a direct correlation between cytotoxicity, DNA breakage, and murine-derived topoisomerase II inhibition activities among the podophyllotoxin analogues. See Minocha, A., and Long, B., Biochem Res. Comm., 122, 165 (1984), which is herein specifically incorporated by reference. The isolation and purification of human type II topoisomerase from lymphocytic leukemia cells has provided the means to use this enzyme as a target to investigate the structure-activity relationships among etoposide and related congeners.
It has been shown that the substitution of etoposide's glycosidic moiety by an 4-alkoxy group, as in 4'-demethyl-epipodophyllotoxin ethyl ether, preserves the inhibitory activity of DNA topoisomerase II intact at higher concentrations. See Thurston, L. S., Irie, H., Tani, S., Han, F. S., Liu, Z. C., Cheng, Y. C., and Lee, K. H., J. Med. Chem., 29, 1547 (1986), which is herein specifically incorporated by reference. However, it has also been shown that a series of 4-acyl congeners are less active, even though some of them possessed potent cytotoxicity. See Thurston, L. S., Imakura, Y., Haruna, M., Li, D. H., Liu, Z. C., Liu, S. Y., Cheng, Y. C., and Lee, K. H., J. Med. Chem., 31, (1988), which is herein specifically incorporated by reference.